Image guided radiation therapy (IGRT) uses images acquired before a treatment session to guide the application of therapeutic radiation during a treatment session. The goal of an IGRT system is to deliver precise dose of radiation to a stationary or moving target inside the patient and avoid irradiating organs-at-risk. This often includes generating a treatment plan that accounts for target motion before a treatment session, as well as gating or motion tracking during a treatment session (e.g., during radiation delivery). The treatment plan typically contains a fluence map or a segmented fluence map that accounts for radiation therapy machine geometry and a list of firing positions (also known as control points, which may be any locations from which a therapeutic radiation source may apply radiation to a patient), from which instructions to the radiation therapy system may be generated, so that the therapeutic radiation source emits radiation that generates a dose distribution that approximates or matches the dose distribution specified by the treatment plan. The machine instructions may include, for example, a set of beam intensities to be applied to the patient at each firing position (or a control point), and/or beam-limiting device instructions (e.g., multi-leaf collimator or MLC instructions) that indicate the MLC configuration for each of those firing positions. Some radiation therapy systems may have integrated imaging systems to aid in target motion tracking during a treatment session. Some treatment plans may generate and store the machine instructions directly, without generating a fluence map initially. However, it is generally understood that a set of machine instructions can be equivalently represented by a segmented fluence map together with radiation therapy machine geometry and a list of firing positions or control points.
Currently, radiation therapy systems that track the motion of a target region, do so by acquiring a high-SNR or high-resolution image of a target region using an imaging system, determining the location of the centroid of the target region, shifting the treatment plan fluence map according to backprojected shifts in the location of the target centroid, and delivering dose based on the shifted fluence map. That is, if a radiation target has shifted to the left (or right), the treatment plan fluence map is shifted such that the generated beam also shifts to the left (or right). These steps may be repeated in the duration of a treatment session. Some systems may use the changing target centroid location over time to generate a motion model to predict the future location of a target region, which may be used to gate the radiation beam delivery. Some systems create motion models that correlate surrogate marker motion with internal tumor motion, and use predicted tumor location (based on surrogate marker location) to shift the fluence maps.
However, these methods require that the acquired images Xi are high-quality, high-SNR full or complete images (e.g., sufficient contrast data between the radiation target and the background of the image), otherwise the target centroid cannot be accurately calculated. Shifting a treatment plan's fluence map based on an imprecise or inaccurate target centroid location data may not improve the efficacy of the treatment plan, and may still result in the irradiation of non-target regions. Since high-quality images sufficient for centroid calculations often require several minutes of data acquisition, or cannot be acquired at sufficiently high frame rates (at least 1 or 2 images per second) due to imaging dose concerns (as in X-ray radiography or tomography), such images acquired during a treatment session do not provide real-time target location data upon which the radiation therapy system may act. The inability to update or correct the delivery of radiation to account for patient and/or tumor motion in real-time may result in radiation delivery to non-target regions.